The physiological consequences of laser micro-irradiation of cells has been investigated using a variety of pump-probe and micro-spectrofluorometric techniques. Fluorescent membrane probes have been used as optical thermometers to measure the temperature of a single living cell held in an optical trap and to observe transient temperature rises when cells are exposed to pulsed infrared laser micro-irradiation. It has also been observed that pulsed infrared laser micro-irradiation causes a decrease in the cellular pH, as measured by an intracellular pH-sensitive dye.